Dinuclear nonheme iron enzymes

Cyclic voltammetry has been also used for estimation of the rate constants for oxidation of water-soluble ferrocenes in the presence of HRP (131). There is a perfect match between the data obtained spectrophotometrically and electrochemically (Table IV), which proves that the cyclic voltammetry reveals information on the oxidation of ferrocenes by Compound II. It is interesting to note that an enzyme similar to HRP, viz. cytochrome c peroxidase, which catalyzes the reduction of H202 to water using two equivalents of ferrocytochrome c (133-136), is ca. 100 times more reactive than HRP (131,137). The second-order rate constant equals 1.4 x 106 M-1 s 1 for HOOCFc at pH 6.5 (131). There is no such rate difference in oxidation of [Fe(CN)e]4- by cytochrome c peroxidase and HRP (8). These comparisons should not however create an impression that the enzymatic oxidation of ferrocenes is always fast. The active-R2 subunit of Escherichia coli ribonucleotide reductase, which has dinuclear nonheme iron center in the active site, oxidizes ferrocene carboxylic acid and other water-soluble ferrocenes with a rate constant of... 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

The goal of diiron model chemistry is to develop small molecule systems that accurately reproduce spectroscopic, structural, and more ambitiously, reactivity aspects of driron metaUoproteins. Despite being structurally similar, diiron enzymes carry out a variety of catalytic processes see Iron Proteins with Dinuclear Active Sites)Advancements in the synthesis and characterization of small molecule mimics for nonheme diiron enzymes have been tremendous in the last decade. Biomimetic studies have been carried out in efforts to reproduce the structural and functional aspects of these biocatalysts. Although this has been a challenging endeavor, much information regarding the structural and mechanistic aspects of catalytic intermediates has been obtained. 

NOR has been suggested to contain a heme c and two heme b groups [279-285], so that this type of enzyme has been defined as a cytochrome cbbNo complex with the heme bNo at a binuclear catalytic site along with a nonheme iron [276]. It has been proposed [281], by similarities between NOR and the family of cytochrome oxidase, that the catalytic site for NO binding and activation is the dinuclear high spin heme b and the nonheme containing iron (III), and that low-spin hemes are used as electron sinks. 

The catalases catalyze the disproportionation of hydrogen peroxide (equations). Most catalases contain the iron-protoporphyrin IX prosthetic group see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases). However, some bacteria are able to synthesize catalases that are not inhibited even by millimolar concentrations of azide and cyanide, suggesting that some catalases are nonheme enzymes it is now known that these enzymes possess a dinuclear Mn active site. 

There are two distinct forms of MMOs one is soluble MMO (sMMO), which contains a nonheme dinuclear iron center, and the other is particulate MMO (pMMO), which is a copper-containing enzyme. Although sMMO has been well characterized and studied (78,79), many aspects of pMMO chemistry and biochemistry remain unclear. Because of the difficulty in the isolation and purification of pMMO, four different research groups have reported various ranges of copper stoichiometries for purified pMMO 2 (80), 2-3 (81) 8-10 (82), and 15-20 (83) copper ions per 100 kDa. The first crystal structure of pMMO firom M. capsulatus Bath (28) shows a heterotrimer of three subunits, and each subunit contains three polypeptides pmoB (a, 47 kDa), pmoA (, 24 kDa), and pmoC (y, 22 kDa). Three types of metal centers were found in pMMO a mononuclear copper, a dinuclear copper center in pmoB, and a zinc center in pmoC. 

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